Key fobs may be paired with a control unit, such as vehicle control unit, to perform well-known actions such as opening/closing and locking/unlocking vehicle doors. The key fob may be capable of transmitting only, which limits the authentication processes available through a challenge message sent from the control unit to verify the commands sent by a key fob due to the inability for two-way communications. A control unit must be able to verify the validity of received commands and to reject unauthorized commands, including replays of earlier transmissions from a valid key fob.
Occasionally, a key fob will be lost or its holder may no longer be authorized to access the control unit. In this situation, there must be a process that allows the control unit to identify which key fobs are still valid and which should be ignored.